1. Field of the Invention
The present invention relates to ophthalmic lenses and, more particularly, ophthalmic lenses having a progressively variable refracting power.
2. Related Background Art
Various types of conventional ophthalmic lenses having a progressively variable refracting power are known as auxiliary ophthalmic lenses for correcting an amplitude of accommodation of an eye when accommodation astheney occurs to fail to observe an object near the eye An ophthalmic lens of this type has an upper distance vision correction area (to be referred to as a portion for distance vision hereinafter), a lower near vision correction area (to be referred to as a portion for myopia hereinafter), and a progressively variable refracting power area (to be referred to as an intermediate portion hereinafter) which is defined between the portions for distance vision and near vision and a refracting power of which is continuously changed.
An intersection line between an object-side lens surface and a cross section along a meridian extending substantially perpendicular to substantially the center of a lens surface of the known ophthalmic lens having a progressively variable refracting power is used as a reference line for specifications such as an additional power which represents a difference between the refracting powers of the portions for distance vision and near vision. The reference line is also used as an important reference line in design of these lenses. In the ophthalmic lenses having asymmetrical portions for near vision upon consideration that when a user wears the ophthalmic lenses, the portions for near vision tend to come close to his nose, a vertical central line which passes through the centers of the portions for distance vision and near vision is defined as a reference line. These reference lines are referred to as "principal meridional curves" hereinafter in the present invention.
Conventional ophthalmic lenses having a progressively variable refracting power have a surface shape having a so-called umbilical line in which microscopic spheres continue along the entire principal meridional curve, and a surface shape which is not umbilical, i.e., two principal radii of curvature perpendicular to each other are equal, in part of the principal meridional curve, but in which radii of curvature perpendicular to each other are different in this part of the curve. In other words, surface shapes on the principal meridional curve are classified into two shapes, i.e., a surface shape which is umbilical, i.e., principal radii of curvature perpendicular to each other are equal along the entire principal meridional curve; and a surface shape which is not umbilical on at least part of the principal meridional curve, but in which a radius of curvature along the principal meridional curve is different from that in a direction perpendicular to the principal meridional curve.
In a paper describing the law of Minkwitz as in Optica Acta, Vol. 10, No. 3, 1963, July, at least an intermediate portion of the principal meridional curve must not be umbilical. More specifically, according to the law of Minkwitz, an astigmatic difference in the refracting power on the surface of the lens is twice a change in refracting power on the surface along the umbilical line in a direction perpendicular to the principal meridional curve. Therefore, it is difficult to widen a clear vision area.
An ophthalmic lens having a progressive variable refracting power, which complies with the above law, is disclosed in Japanese Patent Laid-Open (Kokai) No. 52-110646. In this prior-art invention, a predetermined difference is set between two principal radii of curvature perpendicular to each other in an intermediate portion of the principal meridional curve, and the clear vision area in the intermediate portion can be increased.
Japanese Patent Laid-Open (Kokai) No. 56-78817 discloses an arrangement wherein a difference between a curvature pm in the principal meridional curve and a curvature ps in a direction perpendicular thereto is increased from the portion for distance vision to the portion for near vision, and is then decreased in the portion for near vision. In a portion below the portion for near vision, the curvature .rho.m in the principal meridional direction coincides with the curvature .rho.s in the direction perpendicular thereto, thereby reducing distortion at the center of the portion for near vision.
An arrangement in which part of the principal meridional curve is not umbilical is disclosed in Japanese Patent Laid-Open (Kokai) No. 59-58415. In an embodiment disclosed in this prior-art invention, a curvature ps is kept constant in an area from the center of the portion for distance vision at the lower end thereof or the center of the portion for near vision at the upper end thereof to a peripheral portion of the lens in a direction perpendicular to the principal meridional curve. A curvature .rho.m along the principal meridional curve is gradually decreased, thereby widening the clear vision areas of the portions for distance vision and near vision, and at the same time obtaining a compact, lightweight lens.
With the conventional arrangement described above, it is possible to assure the clear vision area to some extent in consideration of the law of Minkwitz. However, the first arrangement disclosed in Japanese Patent Laid-Open (Kokai) No. 52-110646 aims at assuring the clear vision area in the intermediate portion, the second arrangement disclosed in Japanese Patent Laid-Open (Kokai) No. 56-78817 aims at assuring the clear vision area mainly in the portion for near vision, and the third arrangement disclosed in Japanese Patent Laid-Open (Kokai) No. 59-58415 aims at increasing the clear vision areas of both the portions for distance vision and near vision. The first to third arrangements aim at optimizing any of the three areas required for the ophthalmic lenses having a progressively variable refracting power. These three arrangements are not sufficient to increase the clear vision areas in all three areas, i.e., the portions for distance vision and near vision, and the intermediate portion, and improve visual characteristics in all the areas.
In particular, in a conventional ophthalmic lens having a progressively variable refracting power, the line of sight which passes along the intermediate portion of the principal meridional curve is not perpendicular to the lens surface but is inclined with respect to it. If this area is constituted by an umbilical surface, a surface astigmatic difference occurs. In order to satisfy requirements for comfortable field of view in the intermediate portion and reduce lens aberration such as distortion and blurring of the image in the side area of the intermediate portion, the intermediate portion which is an area for changing the refracting power tends to be increased in recent years. In this tendency, a base curve of the front surface of the lens is moderate, and at the same time, an inclination angle between the refracting surface on the principal meridional curve and the line of sight in the intermediate portion is further increased. As a result, an astigmatic difference becomes a serious problem.